Method of electrodepositing duplex microcrack chromium



United States Patent 3 461 048 METHOD OF ELECTROlBEPOSITlNG DUPLEX MICROCRACK CHROMTUM Henry Mahistedt, Old Greenwich, Conn, and James W.

Dunn, Birmingham, Robert W. Couch, Royal Oak, and

Edgar .l. Seyb, Jr., Oak Park, Mich., assignors to M&T

Chemicals lnc., New York, N.Y., a corporation of Delaware N0 Drawing. Continuation-impart of application Ser. No.

816,371, May 28, 1959. This application Feb. 27, 1963,

Ser. No. 261,526

Int. Cl. (323%: 5/50, 5/46, 5/06 US. Cl. 20441 This application is a continuation-in-part of application Ser. No. 816,371 filed May 28, 1959, now abandoned.

The present invention is directed to a novel process for electrodepositing chromium and to the novel chromium deposits formed thereby.

Chromium which is widely used as a decorative and protective surface finish has usually been electrodeposited in too thin a layer to be of substantial value for protecting the basis metal. To provide corrosion resistance with a decorative chromium surface, copper and nickel are usually deposited on the basis metal before the final decorative chromium. The corrosion resistance of such a composite is almost wholly attributed to the properties of the undercoat. A continued need exists in the trades, particularly in the automobile industry, for chromium surfaced electrodeposits having improved corrosion resistance.

9 Claims It is an object of the present invention to provide a n process for electrodepositing corrosion resistant chromium surfaces.

It is another object of this invention to provide a process for electrodepositing composites, copper nickel layers surfaced with a chromium electrodeposit, having greatly improved corrosion resistance.

Other objects and advantages will become apparent from the following description.

We discovered that by electrodepositing a duplex chromium deposit under specified conditions, it is possible to obtain an electrodeposit having superior corrosion resistance. The first layer of chromium deposited on the basis metal is deposited from a bath having superior covering power. These are the chromic acid (hereinafter referred to as CrO baths containing from about 150 g./l. to about 525 g./1. and preferably 200 g./l. to 400 g./l. of CrO and having only sufiicient catalyst ions so that the ratio of the weight of CrO to the weight of the catalyst ions (hereinafter referred to as ratio) is between 110:1 and 175:1 and preferably between 120:1 to 150:1. The plating temperature may generally vary between 32 C. and 60 C. and is preferably between about C. and 57 C. The current density utilized is conventional and varies between 4 amp/sq. drn. and 60 amp/sq. dm., depending primarily on the plating temperature. These current densities are conventional and are described in Chromium Plating by Morisset et 211., published by Robert Draper Ltd., 1954, pages 351-363, and particularly, page 354. The baths are catalyzed by the presence of sulfate ions and fluoride-containing ions such as fluosilicate, fluozirconate, fiuoaluminate, and fluotitanate. Of the mixed sulfate and fluoride-containing catalyst between 15% and 80%, and preferably between 25% and of the catalyst should be fluoride-containing catalyst ions, the remainder being sulfate ions. For purposes of calculating ratio, the amount of the complex fluoride, such as fiuosilicate, present is halved and added to the amount of sulfate present.

The outer layer of chromium is electrodeposited on the lower layer. The lower layer must be clean when the outer layer is applied. This may be achieved by moving Patented Aug. 12, 1969 the piece from the tank in which the first layer is applied immediately to the tank in which the second (outer) layer is applied. If there is a significant time interval before the entry of the piece into the bath for plating the second layer, it should be cleaned and activated by conventional methods. The second layer is applied from a sulfate and fluoride-containing CrO bath which is operated in such fashion to produce cracked electrodeposits and preferably highly cracked deposits.

The fluoride-containing ions may be supplied in the form of such complex fluorides as fluosilicate, fluozirconate, fluotitanate, and fluoaluminate. Of the mixed sulfate and fluoride-containing catalyst, between 15% and and preferably between 35% and 75% of the catalyst should be fluoride-containing catalyst ions, the remainder being sulfate ions. For purposes of calculating ratio, half the amount of complex fluoride ions is added to the amount of sulfate ions present, as noted hereinbefore. To be useful in this application, the CIO;, bath should have a ratio of between 50:1 to :1 and preferably between about 65:1 to about 90:1. The CrO concentration is in the range of g./l. to 450 g./l. and preferably between g./l. to 300 g./l. Electrodeposition is carried out at temperatures between about 32 C. to 60 C. and preferably between 43 C. and 55 C. Conventional current densities are used as noted hereinbefore. The upper chromium layer should be highly cracked, preferably having more than 40 cracks per cm. However, satisfactory results from duplex chromium electrodeposit-s have been obtained where the upper layer contained as few as 10 cracks per cm. For a specific bath, under constant process conditions, the greater the thickness of deposit, above about .50.75 micron, the greater the number of cracks per cm. The number of crack lines per cm. is determined by microscopically examining the surface of a specimen at reasonably high magnification, e.g., 100x An arbitrary straight line one cm. long is selected in the microscopic field and the average number of crack-lines which cross such arbitrary line are counted.

When plating complex shapes in a given bath, the thickness of electrodeposit obtained under specific conditions varies in different sections of the piece, dependent on the cathode current density on each section; the low current density areas having the least thickness of deposit. When determining suitable conditions for applying useful deposits, consideration is given to obtaining an average desired thickness on the major significant areas of the piece being plated, while at the same time obtaining sulficient deposit in the low current density areas to provide minimum adequate corrosion protection. Generally, the thicker the deposit, the greater the corrosion protection achieved. For purposes of achieving superior corrosion resistance in this novel duplex chromium, it is desirable that the first layer have an average thickness in the sig nificant areas of at least 0.38 micron, and preferably at least 1 micron. However, satisfactory results have been obtained where the minimum thickness in the low current density areas is as little as 0.25 micron. The upper electrodeposit should have an average thickness of at least 0.5 micron in the significant areas. A thickness of at least 1.0 micron is preferable for this layer. The total thickness of the duplex layer is preferably in excess of 1.5 microns, in significant areas to achieve corrosion resistance satisfactory for many purposes. Superior corrosion resistance has been achieved when the duplex electrodeposit is in excess of 3.8 microns. Significantly improved corrosion results are achieved when the total thickness of the duplex layer is as low as 1 micron.

Chromium plating baths are frequently designated as chromic acid baths. Herein the chromic acid content of the bath is referred to as Cr0 (more accurately designated 3 chromic anhydride). The bath may be made up by supplying CrO in the form of chromic anhydride or in the form of compounds containing cations which do not adversely affect the bath characteristics; such compounds include the chromates, dichromates, and polychromates of potassium, sodium, magnesium, and calcium. The CrO may also be added in the form of chromic acid and/or dichromic acid in solution.

The catalyst ions may be incorporated in the bath by addition in the form of the respective acid and/or in the form of a salt with a cation that does not adversely affect bath characteristics, such as potassium, sodium, calcium, strontium, magnesium, chromium, etc. The amount of sulfate ion and of fluoride-containing ion added to the bath and maintained in the bath must be such that their sum results in a ratio conforming to the limits specified hereinhefore. The desired catalyst iOn concentration may be attained by adding soluble salts and controlling concentration by analysis. Baths catalyzed with a mixed sulfate and fluoride-containing catalyst are preferably prepared and maintained from salts which have solubility characteristics such that when excess amounts are added to the bath suflicient amounts dissolve to produce the desired ratio (often lowering the solubility further by the presence of suppressor salts). Self-regulated and controlled baths may be prepared from such salts as strontium sulfate and potassium silicofluoride.

Baths catalyzed with sulfate ion and fluoride-containing ions and operated at temperatures above about 45 C. result in the first layer of chromium being in a substantially crack-free condition, dependent on control of other variables. These baths have excellent covering power and when used as indicated, followed by a second layer electrodeposited as specified hereinbefore, results in a bright duplex chromium deposit with good coverage, substantial thickness in recesses and fine crack pattern over the larger high current density areas of complex parts.

It is preferred to utilize sulfate and fluoride-catalyzed baths prepared largely from CrO sulfate and silicofluoride, having self-regulating catalyst ion characteristics, of the type disclosed and claimed in US. Patents Nos. 2,640,021 and 2,640,022.

For purposes of giving those skilled in the art a better understanding of the invention, the following illustrative examples are given. Test pieces Were plated as noted in the table and tested for corrosion resistance by (i) the Copper Accelerated Salt Spray Test (CASS); or (ii) the Corrodkote Test (CGRK); both described in Pinner, W. L., Plating 44, 763 (1957). The ratings noted are those determined by the ASTM Method, Pray, H. A., Proc. ASTM, 49, 226 (1949).

It will be obvious from inspection of the table that the novel products prepared by the instant invention are superior. It will be further apparent that presence of the bright decorative outer coat of this invention permits attainment of superior corrosion resistance. For example, by comparison of Examples III and IV, it will be observed that when the thickness of the first layer is maintained constant and the thickness of the outer layer of chromium is increase from 0.63 micron to 1.91 microns, the corrosion resistance of the coat increased substantially from 6 CORK cycles of 23 hours to 251 CASS hours.

This same outstanding ability of the outer layer to provide superior resistance to corrosion may be observed by comparison of Examples 1V and V. For fixed total thickness, the outer layer of chromium plate, deposited in accordance with this invention, was increased from 0.63 micron (Example V) to 1.91 microns (Example IV) to thereby effect an unexpected increase in the CASS time from 181 to 251 hours. This is a very significant improvement.

A further comparison showing the outstanding results obtained by the use of the outer layer of chromium plated by the novel process of this invention may be observed by a comparison of Examples VI and VII wherein an increase in the thickness of the outer layer from 1.27 microns to 3.81 microns resulted in an increase in the CORK rating from 6 cycles to 19 cycles.

The sulfate is usually supplied to the bath as strontium sulfate; the fiuosilicate (SiF as the potassium or sodium salt. Replacement of the fiuosilicate compound, in whole or in part, by such salts as sodium fluoaluminate, ammonium fluoaluminate, potassium fluozirconate, and hydrated potassium fluotitanate provide similar duplex chromium deposits having excellent corrosion resistance.

Although greatest corrosion resistance is exhibited by composite electrodeposits in which the duplex chromium of this invention is plated on an undercoat, it is possible to obtain commercially useful duplex chromium electrodeposits directly on such basis metals as steel, nickel and nickel alloys, copper and copper alloys, etc. The basis metal for use under a copper and/ or nickel undercoat includes the wide range of conventionally plated metals such as steels, copper and copper alloys, nickel and nickel alloys, zinc and zinc alloys, etc.

The novel products prepared by the instant process may, according to one embodiment, be article which thus comprise a basis metal with a plate thereon, said plate being characterized by its bright decorative finish and its excellent resistance to corrosion, said plate including as an undercoat a layer of nickel, preferably having a thickness of at least about 20 microns, more preferably of about TABLE OF EXAMPLES Chromium Plating Thicknosses, microns Corrosion Test Result Number (31-03, S04, SiF amp/sq. Temp, 0r, Basis Total, cracks ASTM Layer g./l. g./l. g./l. dm. C. micron metal Cu Ni Cr per cm. Test Hours rating Example I g :8 Zinc. 20.3 20.3 2.54 118 CASS 400 0 II J i 3g Steel 33 .0 1.78 so CORK 7 III g g Zinc... 10.1 20.3 1.20 16 CORK 6 IV fg :3 g :g do 10.1 20.3 2.54 200 CASS 251 s v g g -g}-.-d0. 10.1 20.3 2.54 20 CASS 181 6 VI :8 do 20.3 20.3 2.54 CORK 6 VII 2 i3 d0 10.1 20.3 5.08 470 CORK 8 VIII g 2:23.110"- 20.3 20.3 5.08 340 CASS 483 9 1 6 cycles of 16 hrs. 2 6 cycles of 23 hrs.

3 6 cycles. 4 19 cycles or 23 hrs."

20-35 microns, and as an outer coat a layer of chromium at least 0.5 micron and less than about 5 microns thick and containing at least 10 cracks per cm. According to one embodiment, the outer coat of the plate may include a layer of chromium having a thickness of at least 0.25 micron and a second layer containing at least 10 cracks per cm. and having a thickness of at least 0.5 micron, the total thickness of the chromium plates being at least about 1 micron and less than 5 microns.

As many embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention includes all such modifications and variations as come within the scope of the appended claims.

We claim:

1. A process for electrodepositing bright decorative corrosion resistant duplex chromium on a basis metal comprising (1) electrodepositing a first layer of chromium to a thickness of at least 0.25 micron from a solution having a CrO concentration between 150 g./l. to 525 g./1., and containing sulfate and fluoride-containing catalyst ions in an amount such that the ratio of CrO to that of the catalyst ions is between 110:1 and 175:1 with the sulfate ions being between 20% to 85 of the total catalyst ions, at a temperature between 32 C. and 60 C.; and then (2) electrodepositing n the first layer of chromium a layer of cracked chromium having at least 10 cracks per cm. to a thickness of at least .5 micron from a solution having a CrO concentration between 150 g./1. to 450 g./ 1., and containing sulfate and fluoride-containing catalyst ions in an amount such that the ratio of CrO to that of the catalyst ions is between 50:1 and 10011 with the sulfate ions being between 20% to 85% of the total catalyst ions, at a temperature between 32 C, and 60 C.; to yield a duplex chromium deposit having a total thickness of at least 1 micron and less than about microns.

2. The process of claim 1 in which the first layer of chromium is electrodeposited to a thickness of at least 0.38 micron and the top layer is electrodeposited so that it has at least 40 cracks per cm.

3. The process of claim 1 in which the fluoride-containing catalyst ion is fiuosilicate.

4. The process of claim 1 in which the fluoride-containin g catalyst ion is fluoaluminate.

5. The process of claim 1 in which the fluoride-containing catalyst ion is fluotitanate.

6. The process of claim 1 in which the fluoridecontaining catalyst ion is fiuozirconate.

7. A process for electrodepositing corrosion resistant duplex chromium on a basis metal comprising (1) electrodepositing a first layer of chromium to a thickness of at least 0.25 micron from a solution having a CrO concentration between 200 g./ 1. to 400 g./ 1., and containing sulfate and fluoride-containing catalyst ions in an amount such that the ratio of CrO to that of the catalyst ions is between 120:1 and 150:1 with the sulfate ions being between 35% to 75% of the total catalyst ions, at a temperature between 40 C. and 57 C.; and then (2) electrodepositing on the first layer of chromium a layer of cracked chromium having at least cracks per cm. to a thickness of at least 0.5 micron from a solution having a CrO concentration between 175 g./1. to 300 g./1., and containing sulfate and fluoride-containing catalyst ions in an amount such that the ratio of CrO to that of the catalyst ions is between 65:1 and 90:1 with the sulfate ions being between 25% and 65% of the total catalyst ions, at a temperature between 43 C. and 55 C.; to yield a duplex chromium deposit having a total thickness of at least 1 micron and less than about 5 microns.

8. The process of claim 7 in which the first layer of chrpmium is electrodeposited to a thickness of at least .38 micron and the top layer is electrode'posited so that it has at least 40 cracks per cm.

9. A process for electrodepositing corrosion resistant duplex chromium on a basis metal comprising (1) elecrodepositing a first layer of chromium to a thickness of at least 0.38 micron from a solution having a CrO concentration between 200 g./ 1. to 400 g./ 1., and containing sulfate and fluoride-containing catalyst ions in an amount such that the ratio of CrO to that of the catalyst ions is between 120:1 and 15021 with the sulfate ions being between 35% to 75 of the total catalyst ions, at a temperature between 40 C. and 57 C. and then (2) electrodepositing on the first layer of chromium a layer of cracked chromium having at least 40 cracks per cm. to a thickness of at least 1 mircon, from a solution having a CrO concentration between 175 g./l. to 300 g./l., and containing sulfate and fluoride-containing catalyst ions in an amount such that the ratio of CrO to that of the catalyst ions is between 65:1 and 9011 with the sulfate ions being between 25% and 65 of the total catalyst ions, at a temperature between 43 C. and 55 C.; to yield a duplex chromium deposit having a total thickness of at least 1.5 microns and less than about 5 microns.

10. The process of claim 9 in which the first layer of chromium is electrodeposited to a thickness of at least 1 micron and the total thickness of the duplex deposit is at least 3.8 microns.

11. The process of claim 9 in which the fluoride-containing catalyst ion is fiuosilicate.

12. The process of claim 9 in which the fluoride-containing catalyst ion is fluoaluminate.

13. The process of claim 9 in which the fluoride-containing catalyst ion is fluotitanate.

14. The process of claim 9 in which the fluoride-containing catalyst ion is fiuozirconate.

15. A method of forming a multi-layer decorative chromium coating which comprises electrodepositing a nickel coating onto the surface of a metal part, electrodepositing a first bright decorative chromium layer onto the surface of said part under an average cathode current density of approximately 4 amperes per sq. dm. to 60 amperes per sq. dm. from a bath at a temperature of approximately 32 C. to 60 C. containing approximately 150 g./1. to 525 g./l. CrO and having sufficient catalyst ions so that the ratio of the weight of CrO to the weight of catalyst ions is between 110:1 and 175:1, and then, without intervening abrasive treatment of said chromium plated surface and under an average cathode current density of approximately 4 amperes per sq. dm. to 60 amperes per sq. dm. electrodepositing onto said first layer a second decorative chromium layer which has a denser crack pattern than said first layer using a bath at a temperature of approximately 32 C. to 60 C. containing about 150 g./l. to 450 g./l. CrO and having sufficient catalyst ions so that the ratio of the weight of CrO to the weight of catalyst ions is between 50:1 to :1.

16. The method of forming a highly durable multilayer decorative chromium plated article which comprises successively electrodepositing a nickel coating and two different decorative chormium layers without intervening abrasive treatment of the first chromium plated layer before application of the second chromium layer, wherein the first of said layers is electrodeposited from an aqueous bath solution which is at a temperature between 32 C. and 60 C., said bath comprising essentially between 150 grams per liter and 525 grams per liter of CrO a total catalyst ion concentration of between 2.9 grams per liter and 3.5 grams per liter of sulfate and silicofluoride ions, the ratio of CrO concentration to total catalyst ion concentration being between :1 and 175 :1, the sulfate ion content being between 20% and 85% of the total catalyst concentration, and the second of said layers is electrodeposited from an aqueous chromium plating bath containing about grams to 450 grams per liter of CrO two catalyst-supplying compounds, and a soluble, non-catalytic compound, one of said catalyst-supplying compounds being strontium sulfate in an amount sufiicient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, the other of said catalyst-supplying compounds being an alkali metal silicofluoride in an amount sufiicient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, said non-catalytic compound being a strontium compound in an amount sufficient to suppress the concentration of said strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter compound to a lower but greater than zero concentration.

17. The method of forming a highly durable multilayer decorative chromium plated article which comprises successively electrodepositing a nickel coating and at least two different decorative chromium layers without intervering abrasive treatment of the first chromium plated layer before application of the second chromium layer, wherein the first applied of said layers is electrodeposited from an aqueous bath solution which is at a temperature between 32 C. and 60 C., said bath comprising essentially between 150 grams per liter and 525 grams per liter of CrO a total catalyst ion concentration of sulfate and silicofluoride ions to achieve a ratio of CrO concentration to total catalyst ion concentration between 110:1 and 175:1, the sulfate ion content being between 110:1 and 175 :1, the sulfate ion content being and the second applied to said layers is electrodeposited from a bath solution containing about 150 grams to 450 grams per liter of CrO two catalyst-supplying compounds, and a soluble, non-catalytic compound, one of said catalyst-supplying compounds being strontium sulfate in an amount sufficient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, the other of said catalyst-supplying compounds being an alkali metal silicofiuoride in an amount sufiicient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, said non-catalytic compound being a strontium compound in an amount sufiicient to suppress the concentration of said strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter compound to a lower but greater than zero concentration.

18. The method of forming a highly durable decorative chromium coating which comprises successively applying to the surface of a metal part a copper coating, a nickel coating and a multi-layer decorative chromium coating having successive layers applied without intervening abrasive treatment in which the first applied of said layers is electrodeposited from an aqueous chromium plating bath solution at a temperature between 32 C. and 60 C. said bath comprising essentially between 150 grams per liter and 525 grams per liter of CrO a total catalyst ion concentration of between 2.9 grams per liter and 3.5 grams per liter of sulfate and silicofluoride ions, the ratio of CrO concentration to total catalyst ion concentration being between 110:1 and 175 :1, the sulfate ion content being between and 85% of the total catalyst concentration, the sulfate in said bath being added in the form of strontium sulfate and silicofluoride added in the form of potassium silicofluoride, each in an amount sufficient to saturate said bath and to provide therein an undissolved residue of strontium sulfate and potassium silicofluoride respectively, and a soluble non-catalytic potassium compound to suppress the concentration of silicofluoride ions, the second applied of said layers being deposited from a chromium plating bath solution containing about 150 grams to 45 0 grams per liter of CrO two catalyst supplying compounds, and a soluble, non-catalytic compound, one of said catalyst-supplying compounds being strontium sulfate in an amount suflicient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, the other of said catalystsupply compounds being an alkali metal silicofluoride in an amount suflicient to saturate said bath and to provide an undissolved residue of alkali metal silicofiuoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, said non-catalytic compound being a strontium compound in an amount sufiicient to suppress the concentration of said strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter compound to a lower but greater than zero concentration.

19. The method of forming a highly durable decorative chromium coating which comprises successively applying to the surface of a metal part of copper coating, a nickel coating and a multi-layer decorative chromium coating having successive layers, at least about 0.5 micron in thickness, applied without intervening abrasive treatment in which the first applied of said layers is electrodeposited from an aqueous chromium plating bath solution at a temperature between 32 C. and 60 C., said bath comprising essentially between 150 grams per liter 'and 525 grams per liter of CrO a total catalyst ion concentration of sulfate and silico-fluoride ions to achieve a ratio of CrO concentration to total catalyst ion concentration between :1 and 175:1, the sulfate ion content being between 20% and 85% of the total catalyst concentration, and the second applied of said layers being deposited from an aqueous bath solution containing about grams to 450 grams per liter of CrO two catalyst supplying compounds, and a soluble, non-catalytic compound, one of said catalyst-supplying compounds being strontium sulfate in an amount sufiicient to saturate said bath and to provide an undissolved residue of strontium sulfate in the bath, the other of said catalyst-supplying compounds being an alkali metal silicofluoride in an amount suflicient to saturate said bath and to provide an undissolved residue of alkali metal silicofluoride in the bath, said alkali metal being selected from the class consisting of potassium and sodium, said non-catalytic compound being a strontium compound in an amount sufficient to suppress the concentration of said strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter compound to a lower but greater than zero concentration.

References Cited UNITED STATES PATENTS 2,678,908 5/ 1954 Tucker 204-41 2,755,537 7/1956 Smart 29196.6 2,800,438 7/1957 Stareck et a1. 204-41 2,871,550 2/1959 Weinberg et a1. 29l96.6 2,916,424 12/ 1959 Stareck et al 204-41 3,157,585 11/1964 Durham 20441 JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner U.S. Cl. X.R. 20451 

1. A PROCESS FOR ELECTRODEPOSITING BRIGHT DECORATIVE CORROSION RESISTANT DUPLEX CHROMIUM ON A BASIS METAL COMPRISING (1) ELECTRODEPOSITING A FIRST LAYER OF CHROMIUM TO A THICKNESS OF AT LEAST 0.25 MICRON FROM A SOLUTION HAVING A CRO3 CONCENTRATION BETWEEN 150 G./1. TO 525 G./1., AND CONTAINING SULFATE AND FLUORIDE-CONTAINING CATALST IONS IN AN AMOUNT SUCH THAT THE RATIO OF CRO3 TO THAT OF THE CATALYST IONS IS BETWEEN 110:1 AND 175:1 WITH THE SULFATE IONS BEING BETWEEN 20% TO 85% OF TOTAL CATALYST IONS, AT A TEMPERATURE BETWEEN 32* C. AND 60*C.; AND THEN (2) ELECTRODEPOSITING ON THE FIRST LAYER OF CHROMIUM A LAYER OF CRACKED CHROMIUM HAVING AT LEAST 10 CRACKS PER CM. TO A THICKNESS OF AT LEAST .5 MICRON FROM A SOLUTION HAVING A CRO3 CONCENTRATION BETWEEN 150 G./1. TO 450 G./1., AND CONTAINING SULFATE AND FLUORIDE-CONTAINING CATALYST IONS IN AN AMOUNT SUCH THAT THE RATIO OF CRO3 TO THAT OF THE CATALYST IONS IS BETWEEN 50:1 AND 100:1 WITH THE SULFATE IONS BEING BETWEEN 20% TO 85% OF THE TOTAL CATALYST IONS, AT A TEMPERATURE BETWEEN 32*C. AND 60*C.; TO YIELD A DUPLEX CHROMIUM DEPOSIT HAVING A TOTAL THICKNESS OF AT LEAST 1 MICRON AND LESS THAN ABOUT 5 MICRONS. 